1. Field of the Invention
The invention relates to magnetic compasses, and is especially concerned with automatic calibration of magnetic compasses. The invention is especially, but not exclusively, applicable to magnetic compasses for use with terrestrial vehicles such as automobiles.
2. Description of the Prior Art
Known magnetic compasses used in vehicles comprise a sensor, for example a so-called flux gate sensor or a Hall effect sensor, and an electronic circuit for deriving, from the signals produced by the sensor, the magnitude and direction of the terrestrial magnetic field sensed by the sensor.
If there were no distortions, the terrestrial magnetic field vector displayed by the magnetic compass would sweep out a perfect circle, with its origin corresponding to the datum of the compass, as the vehicle was rotated through a complete circle. In practice, however, when a magnetic compass is used in a vehicle, the magnetic field sensed by the sensor is distorted by what are known as "hard-iron" effects and "soft-iron" effects, which result from permanent and induced magnetism, respectively, of the vehicle.
Soft-iron effect depends primarily upon the geometry of the vehicle and the permeability of the metal it comprises. Soft-iron effect produces a two-cycle error in 360 degrees, which causes the locus swept by the measured magnetic field vector to assume an elliptical pattern with its eccentricity varying as a function of degrees of error. The peak error due to the soft-iron effect might be as much as 20.degree. to 30.degree., though usually it will be less than 10.degree. for most locations on a vehicle.
Hard-iron effects are caused primarily by metal surrounding the sensor acting as a secondary source of emission. Hence, expansion and contraction of the metal surrounding the magnetic compass, together with vibration and other strong magnetic perturbations, cause the origin of the ellipse to deviate as a function of the amplitude and phase of the vector representing the hard-iron perturbation, typically producing a one-cycle error in 360 degrees.
If valid heading readings are to be obtained, the magnetic compass must be calibrated to compensate for these distortions. Manual correction techniques involve the positioning of small magnets around the sensor to compensate for perturbing magnetic fields. This process is fairly complex since the adjustments are made by positioning the vehicle at different angles while a skilled technician adjusts the positions of the small magnets.
Semi-automatic calibration techniques have been proposed which require position errors at different headings to be entered manually into a computer memory which then corrects the outputs from the flux gate sensors.
Various automatic compensation systems have been proposed. U.S. Pat. No. 4,953,305 issued Sep. 4, 1990 (Van Lente et al), and U.S. Pat. No. 4,546,551 issued October, 1985 (Franks) which are incorporated herein by reference, disclose a magnetic compass for a vehicle which comprises a magnetic field sensor of the flux-gate type and a microprocessor. The microprocessor corrects for the hard-iron and soft-iron effects by monitoring the maximum and minimum signals during movement through a complete 360.degree. rotation or path of travel and continuously averaging the data. This method may not be entirely satisfactory where errors are due to tilting of the vehicle, which tend to be relatively slow. It will also tend to be noise sensitive.
U.S. Pat. No. 4,738,031 (Alberter et al), issued April, 1988, which is incorporated herein by reference, discloses a method of calibrating an electronic magnetic compass for soft-iron effects and U.S. Pat. No. 4,989,333 issued Feb. 5, 1991 (Helldorfer et al), which is incorporated herein by reference, discloses a further development of the magnetic compass disclosed in U.S. Pat. No. 4,738,031 which employs fast dynamic compensation or updating of "hard-iron" interfering field changes due to electrical changes caused by connecting or disconnecting loads in motor vehicles. This system of calibration makes use of a locus diagram and "bins" to keep track of the soft-iron and hard-iron perturbations vectors. It involves rotating the vehicle in which the compass is mounted through one or two full circles, determining the maximum and minimum values of the X and Y readings, and deriving the centre point of the elliptical polar frequency of the magnetic field. Thereafter, the semi-axes of the ellipse are established by determining the maximum and minimum values from that centre point. Such a system is not entirely satisfactory because it uses only a few points on the polar diagram and so is inherently sensitive to noise. Moreover, the methods proposed in U.S. Pat. No. 4,738,031 and U.S. Pat. No. 4,989,383 will tend to compensate for other non-permanent effects such as vehicle tilt and short term magnetic perturbations.
An object of the present invention is to at least mitigate the afore-mentioned disadvantages and provide a magnetic compass with improved automatic compensation for distortions.